Objective: To assess the incidence of placental basal plate myometrial fibers in preterm and term gestations and correlate this finding with clinical observations and placental histopathology.
Methods: Placentas from 457 singleton births before 32 weeks' gestation and 108 uncomplicated singleton births after 37 weeks' gestation were examined histopathologically. Pregnancies complicated by maternal chronic hypertension, diabetes mellitus, coagulopathy, placenta previa, stillbirth, multiple fetuses, and fetal congenital anomalies were excluded from both groups. In the preterm group, 158 patients had preterm labor with intact membranes, 192 had preterm premature rupture of membranes (PROM), 31 had placental abruption without hypertension, and 76 had preeclampsia. Histopathology detected the presence of placental basal plate myometrial fibers, placental vascular lesions, and villous damage related to vascular insufficiency.
Results: Forty-four of 457 (9.6%) of preterm placentas had basal plate myometrial fibers, compared with one of 108 (0.9%) term controls (P < .001). Uteroplacental vessels with abnormal physiologic changes were more frequent and placental weights were lower in cases with basal plate myometrial fibers (P < .003 and P < .03, respectively). No other uteroplacental vascular lesions were related to basal plate myometrial fibers. The frequency of placental basal plate myometrial fibers was nine of 76 (12%) in cases complicated by preeclampsia, 21 of 192 (11%) cases of PROM, nine of 158 (5.7%) cases of preterm labor, and four of 31 (13%) cases of placental abruption without hypertension; these frequencies were not significantly different, and there was no significant relationship to gravidity, parity, mode of delivery, or birth weight.
Conclusion: Placental basal plate myometrial fibers occur in ten times as many preterm births as term births. This finding is associated with both abnormal uteroplacental physiologic changes and decreased placental weight, and may explain the increased incidence of abnormalities of the third stage of labor associated with preterm delivery.